Limestone deposits are at the bottom of a coral reef. Limestone and coral are primary components of a coral reef. Coral is made from colonies of small marine animals called polyps. Polyps are connected together by living tissue. Hermatypic polyps are the kinds that create hard coral that secrete limestone. The hardened limestone is what forms the reef. Living polyps live only in the uppermost layer of the reef. The ReefED educational website explains: "As a coral grows new polyps replace old polyps which become the calcareous skeleton."
Polyps die after contributing to what becomes the bottom layer of the coral reef structure. From top to bottom, organisms that make up a coral reef are determined by the surrounding land and marine environment. For example, Goniopora (Galaxea) coral exist in a muddy foundation and inshore waters. Other coral reefs are situated in mangrove forests and seagrass beds that are nearby. Types of coral reefs are fringing reefs, barrier reefs and coral atolls.
The floor of the ocean serves as a bottom for fringing reefs. When a fertilized coral egg develops into a planula, after a while, it settles on the ocean's bottom. Eventually it buds and begins forming the coral colony. The ocean floor of the continental shelf is where the reefs that form along the coastline originate. The bottom there contains sediment from continent erosion. Fringing reefs are the type that remain continuous with the land mass.
Barrier reefs grow farther out in the ocean, but parallel to the shoreline. The Great Barrier Reef consists of 3,000 individual reef systems. Layer after layer of limestone rests on a sandy bottom. According to the Great Barrier Reef Organization, "Many generations of dead coral have built themselves into great walls of stone covered in a diverse range of living organisms..." Limestone platforms at the sandy bottom of the reef lay carpeted with soft coral, sponges and other plants and animals.
Coral atolls are reefs that develop progressively over millions of years. These rings of coral actually start out as fringe reefs that surround a volcanic island. The fringe reefs later form barrier reefs when the island sinks almost to sea level. The reef continues growing upward, even after the island has sunk and is completely beneath sea level. The rocky bottom surface of a coral atoll is called a cay. It is composed of coral that has been broken into a multitude of pieces due to wave and wind erosion.
